1. Field of the Invention
The present invention relates to a method and an apparatus for providing a packet data service in a mobile communication system, and more particularly to a method and an apparatus for transmitting/receiving control information for packet data in a mobile communication system using an Orthogonal Frequency Division Multiple Access (OFDMA) scheme.
2. Description of the Related Art
Active research is being conducted for recent mobile communication systems, in order to use an Orthogonal Frequency Division Multiplexing (OFDM) scheme in high speed data transmission through wire/wireless channels.
The OFDM scheme, which transmits data using multiple carriers, is a special type of a Multiple Carrier Modulation (MCM) scheme in which a serial symbol sequence is converted into parallel symbol sequences and the parallel symbol sequences are modulated with a plurality of mutually orthogonal sub-carriers before being transmitted. That is, in a transmission according to the OFDM scheme in an OFDM system, signals of users are identified by allocating different sub-carriers to physical channels allocated to the users. In general, system resources of an OFDMA system include two dimensional resources including frequency domain resource and time domain resources. That is, when a minimum unit for transmission of packet data through one physical channel on the time axis is called a Transmission Time Interval (TTI), which is also called a frame or a packet transmission interval, the TTI usually includes a plurality of OFDM symbols, each of which includes a plurality of sub-carriers on a frequency axis. Therefore, resources defined in one TTI have a form of two-dimensional resources including a plurality of OFDM symbols on the time axis and a plurality of sub-carriers on the frequency axis.
In the two-dimensional resources, one minimum resource unit, that is, one sub-carrier in one OFDM symbol, is usually called a Time-Frequency bin (TF bin), which servers as a unit for transmission of one modulated symbol in packet data transmission through an actual physical channel.
In a typical OFDM system, a physical channel for packet data transmission is called a packet data channel, and a physical channel for transmission of control information necessary for demodulation of the packet data channel is called a packet data control channel (hereinafter, “control channel”). Usually, packet data and control information are transmitted through the packet data channel and the packet data control channel.
In the OFDM system, a Forward Data CHannel (F-DCH) may be used for transmission of the packet data, and a Forward Shared Control CHannel (F-SCCH) may be used for transmission of the control information.
Packet data control information (hereinafter, “control information”) necessary for demodulation of a packet data channel is transmitted through the packet data control channel.
That is, a receiver receiving packet data through a packet data channel first demodulates the packet data control channel, thereby acquiring control information necessary for demodulation of the packet data channel. Then, the receiver demodulates the packet data channel by using the control information. The transmitted packet data control channel usually includes a user identifier (hereinafter, “Medium Access Control IDentifier (MAC ID)”), resource allocation information, Modulation and Coding Scheme (MCS) information, etc. As used herein, the MAC ID refers to an identifier of an Access Terminal (AT) promised in advance between the AT and an Access Network (AN). In a typical OFDM packet data transmission/reception system, resources are shared by a plurality of users. Further, during every TTI, an Access Network (AN) scheduler selects a user who will use the system resources and transmits packet data through a predetermined resource to the selected user.
When the AN transmits packet data through a packet data channel to a particular AT, a MAC ID is promised in advance between the AN and the AT, and is then transmitted through the packet data channel during every TTI, so as to notify the AT which user the packet data the packet data is forwarded to through the packet data channel. Therefore, every AT within the system continuously monitors the packet data control channel, and continues determining whether a MAC ID received through the packet data control channel is identical with the ATs' own MAC IDs. When a MAC ID received through the packet data channel during a particular TTI is equal to an AT's own MAC ID, the AT determines that a data channel allocated together with the packet data control channel has been allocated to itself, and performs demodulation of the packet data channel.
Next, the control information received through the packet data control channel includes resource allocation information, which refers to information on resources used for transmission of packet data. Since one modulation symbol is carried by one TF bin at the time of packet data transmission through a physical channel as described above, it is impossible to demodulate the packet data channel without information on the resource actually used for the packet data transmission from among the two-dimensional resources within the single TTI, i.e., without information on the TF bin from a receiver. As used herein, MCS information refers to a modulation scheme and a coding rate used for the packet data transmission. The control information includes other type of information according to a particular purpose as well as the above-mentioned information.
FIG. 1 is a block diagram illustrating a structure of a transmitter for constructing a packet data control channel in a typical OFDM-based mobile communication system. Referring to FIG. 1, information bits 100 correspond to the control information as described above, which is assumed to have 34 or 36 bits. The number of bits, which is shown to be 34 or 36 in FIG. 1, is only a particular example and may change according to the communication system.
In the transmitter shown in FIG. 1, a Cyclic Redundancy Check (CRC) bit inserter 102 adds 16 CRC bits for detection of transmission errors in the control information 100 transmitted through the packet data control channel. Further, an encoder tail bit inserter 104 adds encoder tail bits having a size of 8 bits for convolution encoding to the 50 or 52 bits of control information output from the CRC bit inserter 102, and then outputs the encoder tail-added control information to a convolution encoder 106. Then, the convolution encoder 106 performs convolution encoding of the input bits, and a rate matcher 108 then performs rate matching of the encoded output from the convolution encoder 106.
In performing rate matching, the number of output modulation symbols of the control channel is controlled in accordance with the number of total sub-carriers allocated to the packet data control channel and the quantity of the resources allocated to the packet data control channel. The rate matching usually includes steps of repetition and puncturing.
The output of the rate matcher 108 is input to a Quadrature Phase Shift Keying (QPSK) modulator 110, and modulation symbols 112 are generated through a modulation process such as QPSK. Then, the modulated symbols 112 are transmitted through pre-allocated resources.
Meanwhile, packet data may be simultaneously transmitted through a plurality of data channels in a typical OFDMA system. That is, multiple control information elements may be simultaneously transmitted through the control channels as shown in FIG. 1. This implies that an AN may simultaneously transmit packet data to multiple ATs. Therefore, each AT demodulates control information elements through the multiple control channels simultaneously received from the AN, and determines, from the demodulated control information, the control channel through which the same MAC ID as its own MAC ID is received. In a conventional system, in order to support the operation as described above, a maximum number of control channels capable of transmitting the control information is pre-defined and is signaled to all ATs within the system.
For example, when the AN signals to all ATs within the system that “it is possible to transmit a maximum of four control channels,” the ATs perform control channel demodulation based on a premise that each piece of control information can be transmitted through a maximum of four control channels, as long as there is no change in the signaled information. The resource for use in transmission of the control information is determined according to the determined maximum number of transmissible control channels.
FIG. 2 illustrates an example of resource allocation when each piece of control information is simultaneously transmitted through four control channels in a typical OFDM mobile communication system. Referring to FIG. 2, each rectangle indicates one Forward Shared Control CHannel (F-SCCH) block, which is transmitted through a predetermined allocated resource. In FIG. 2, the F-SCCH block size 200 corresponds to a block size for transmission of one control channel block and indicates the number of modulation symbols necessary for transmission of the control channel block.
Usually, the control channel block is transmitted through a plurality of sub-carriers over a plurality of OFDM symbols. FIG. 2 shows only the logical structure of the control channel blocks for convenience of description. However, the control channel blocks may be actually transmitted through physically separated sub-carriers, and the modulation symbols 112, which are the final output in FIG. 1, are transmitted through one control channel block from among the control channel blocks. In the system as described above, in which a predetermined quantity of resources are allocated for transmission of one control channel block and four control channels as an example can be transmitted during one TTI, four control channel blocks are used for transmission of the control information. In the above example, an AT receiving the control information performs demodulation of each of the four control channel blocks through a reverse process to that of the transmitter shown in FIG. 1, and then determines whether each demodulated control channel includes its own MAC ID. That is, in the above example, the AT performs an operation for receiving the control information transmitted through the four control channel blocks.
However, the fact that the maximum number of the transmitted control channels is four does not signify that the control information is always transmitted through four control channels during every TTI. Actually, the number of control channels transmitting control information during each TTI changes according to the scheduling result for the AT by the AN, that is, according to the resource allocation result. For example, if the AT determines to allocate all resources to one AT during a certain TTI, only one control channel is actually used for transmission of the control information during the TTI. However, since the AT does not know the number of control channels actually used for transmission of the control information, the AT attempts the demodulation the same number of times as the maximum number of control channels usable for transmission of the control information during one TTI.
Once the maximum number of control channels used for one-time transmission of the control information has been determined, resources for transmission of as much control information as the maximum number of control channels must be allocated. Further, the resources allocated for the control channel transmission cannot be used for another purpose, even when the number of control channels actually transmitting the control information is less than the maximum number of control channels. Therefore, it may be preferred to limit the maximum number of control channels to a small number. However, if the maximum number of control channels is limited to a small number, that small number may limit the data channel allocation by the AT. Therefore, it is not preferable to limit the maximum number of control channels to a small number. This is because the fact that the maximum number of control channels capable of transmitting control information during one TTI is four as in the above example implies that the maximum number of ATs simultaneously schedulable by the AN is only four.
Meanwhile, the conventional OFDMA system supports multiple layer transmission. The multiple layer transmission implies that a transmitter simultaneously transmits a plurality of packets either to multiple users or to a single user through the same time and frequency resources by using multiple transmission antennas. When the multiple layer transmission as described above is supported, the number of control channels necessary for the multiple layer transmission increases up to the number of the layers, because one control channel should be used whenever the packet data is transmitted through each layer. The increase in the number of control channels as described above results in increasing in the quantity of resources allocated to the control channels. Therefore, such an increase is a waste of resources in a TTI that does not use the resources, while a small maximum number of control channels imposes a limit on resource allocation.